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Avian influenza

From Academic Kids

Avian influenza (also known as bird flu) is a type of influenza virulent in birds. It was first identified in Italy in the early 1900s and is now known to exist worldwide.

Contents

Infection

The causative agent is the avian influenza (AI) virus. AI viruses all belong to the influenza virus A genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses.

Avian influenza spreads in the air and in manure. Wild fowl often act as resistant carriers, spreading it to more susceptible domestic stocks. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat.

Cats are also thought to be possible infection vectors for H5N1 strains of avian flu (Kuiken et al, 2004).

The incubation period is 3 to 5 days. Symptoms in animals vary, but virulent strains can cause death within a few days.

Subtypes pathogenic to humans

Of the 16 subtypes known, only subtypes H5, H7 and H9 are known to be capable of crossing the species barrier from birds to humans. It is feared that if the avian influenza virus undergoes antigenic shift with a human influenza virus, the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global influenza pandemic, similar to the Spanish Flu that killed over 20 million people in 1918 (though a variety of sources quote average figures even higher, up to 100 million in some cases). Many health experts are concerned that a virus that mutates to the point where it can cross the species barrier (i.e. from birds to humans) will inevitably mutate to the point where it can be transmitted from human to human. It is at that point that a pandemic becomes likely. Influenza viruses that infect birds are called avian influenza viruses. Only influenza A viruses infect birds. All known subtypes of influenza A virus can infect birds.

However, there are substantial genetic differences between the subtypes that typically infect both people and birds. Within subtypes of avian influenza viruses there also are different strains (described in Strains). Avian influenza H5 and H7 viruses can be distinguished as low pathogenic and high pathogenic forms on the basis of genetic features of the virus and the severity of the illness they cause in poultry; influenza H9 virus has been identified only in a low pathogenicity form. Each of these three avian influenza viruses (H5, H7, and H9) can theoretically be partnered with any one of nine neuraminidase surface proteins; thus, there are potentially nine different forms of each subtype (e.g., H5N1, H5N2, H5N3 ... H5N9).

H5N1

H5N1 avian influenza strain passed from birds to humans in 1997 in Hong Kong. Eighteen people were infected, of whom six died. The outbreak was limited to Hong Kong. All chickens in the territory were slaughtered.

In January 2004, a major new outbreak of H5N1 avian influenza surfaced in Vietnam and Thailand's poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and mainland China. Intensive efforts were undertaken to slaughter chickens, ducks and geese (over 40 million chickens alone were slaughtered in high-infection areas), and the outbreak was contained by March, but the total human death toll in Vietnam and Thailand was 23 people. In February 2004, avian influenza virus was detected in pigs in Vietnam, increasing fears of the emergence of new variant strains.

Fresh outbreaks in poultry were confirmed in Ayutthaya and Pathumthani provinces of Thailand, and Chaohu city in Anhui, China, in July 2004.

In August 2004 avian flu was confirmed in Kampung Pasir, Kelantan, Malaysia. Two chickens were confirmed to be carrying H5N1. As a result Singapore has imposed a ban on the importation of chickens and poultry products. Similarly the EU has imposed a ban on Malaysian poultry products. A cull of all poultry has been ordered by the government within a 10km radius of the location of this outbreak.

An outbreak of avian influenza in January 2005 affected 33 out of 64 cities and provinces in Vietnam, leading to the forced killing of nearly 1.2 million poultry. Up to 140 million birds are believed to have died or were killed because of the outbreak.

Vietnam and Thailand have seen several isolated cases where human-to-human transmission of the virus has been suspected. In one case the original carrier, who received the disease from a bird, was held by her mother for roughly 5 days as the young girl died. Shortly afterwards, the mother became ill and perished as well. In March, 2005 it was revealed that two nurses who had cared for avian flu patients have tested positive for the disease.

As of April 17, the outbreak had claimed at least 50 human lives — mostly in Vietnam. What concerns health researchers now is that the virus mortality rate in Vietnam has dropped significantly lately, from more than 65% to about 35% in a year. This might be a sign that the virus is able to infect a larger number of people (i.e., the virus is able to spread more easily) and possibly develop into a global pandemic with millions of deaths despite the lower reported percentage of deaths. For example, the mortality rate of 1918 Spanish flu pandemic was less than 5% [1] (http://www.boston.com/news/world/asia/articles/2005/04/24/drop_in_virus_mortality_rate_portends_new_danger?pg=2).

In May 2005, the occurrence of Avian influenza in pigs in Indonesia was reported ("swine flu"). Along with the continuing pattern of virus circulation in poultry, the occurrence in swine raises the level of concern about the possible evolution of the virus into a strain capable of causing a global human influenza pandemic.

  • Potentially nine different subtypes
  • Can be highly pathogenic or low pathogenic
  • H5 infections have been documented among humans, sometimes causing severe illness and death

H7N3

In North America, the presence of avian influenza strain H7N3 was confirmed at several poultry farms in British Columbia in February 2004. As of April 2004, 18 farms had been quarantined to halt the spread of the virus. Two cases of humans with avian influenza have been confirmed in that region.

  • Potentially nine different subtypes
  • Can be highly pathogenic or low pathogenic
  • H7 infection in humans is rare, but can occur among persons who have close contact with infected birds; symptoms may include conjunctivitis and/or upper respiratory symptoms

H9Nx

  • Documented only in low pathogenic form
  • Three H9 infections in humans have been confirmed

Prevention and treatment

Avian influenza in humans can be detected with standard influenza tests. However, these tests have not always proved reliable. In March 2005, the World Health Organization announced that seven Vietnamese who initially tested negative for bird flu were later found to have carried the virus. All seven have since recovered from the disease.

Antiviral drugs are sometimes effective in both preventing and treating the disease, but no virus has ever been really cured in medical history. Vaccines, however, take at least four months to produce and must be prepared for each subtype.

Further, as a result of widespread use of the antiviral drug amantadine to vaccinate chickens in China starting in the late 1990's, the avian flu virus in Asia has developed drug resistance against amantadine [2] (http://www.washingtonpost.com/wp-dyn/content/article/2005/06/17/AR2005061701214.html) . This use of amantadine for poultry goes against international livestock regulations, but China kept it secret until recently, in a manner reminiscent of the secrecy around the early spread of SARS. Chickens in China have received an estimated 2.6 billion doses of amantadine since early 2004.

Increasing virulence

In July 2004 a group of researchers led by H. Deng of the Harbin Veterinary Research Institute, Harbin, China and Professor Robert Webster of the St Jude Children's Research Hospital, Memphis, Tennessee, reported results of experiments in which mice had been exposed to 21 isolates of confirmed H5N1 strains obtained from ducks in China between 1999 and 2002. They found "a clear temporal pattern of progressively increasing pathogenicity". [3] (http://www.pnas.org/cgi/content/abstract/0403212101v1)

Symptoms

In humans, it has been found that avian flu causes similar symptoms to other types of flu:

Taken from: http://www.bupa.co.uk/health_information/html/health_news/270104avianflu.html

Pandemic threat and preparedness plans

The World Health Organisation (WHO) warns that there is a substantial risk of an influenza pandemic within the next few years. One of the strongest candidates is the A(H5N1) subtype of Influenza virus. See "Assessing the pandemic threat" at [4] (http://www.who.int/csr/disease/influenza/en/). WHO published a first edition of the Global Influenza Preparedness Plan in 1999, and updated it in April 2005. See [5] (http://www.who.int/csr/disease/influenza/pandemic/en/index.html) and [6] (http://www.who.int/csr/resources/publications/influenza/WHO_CDS_CSR_EDC_99_1/en/) which define the responsibilities of WHO and national authorities in case of an influenza pandemic. This is the first time a pandemic has been anticipated and is being prepared for.

The aims of such plans are, broadly speaking, the following:

  • Before a pandemic, attempt to prevent it and prepare for it in case prevention fails.
  • If a pandemic does strike, to slow its spread and allow societies to function as well as possible.

Strategies to prevent a pandemic

If avian influenza remains an animal problem with limited human-to-human transmission it is not a pandemic, though it continues to pose a risk.

To prevent the situation from progressing to a pandemic, the following strategies have been put forward:

  • Culling and vaccinating poultry
  • Limiting travel in areas where the virus is found

Strategies to slow down a pandemic

  • Vaccines. A vaccine probably would not be available in the early stages of a pandemic [7] (http://www.cdc.gov/flu/avian/gen-info/pandemics.htm). Once a potential virus is identified, it normally takes at least several months before a vaccine becomes widely available, as it must be developed, tested and authorised. The capability to produce vaccines varies widely from country to country; in fact, only 15 countries are listed as "Influenza vaccine manufacturers" acording to the World Health Organisation [8] (http://www.who.int/csr/disease/influenza/manulist/en/index.html). It is estimated that, in a best scenario situation, 750 million doses could be produced each year, whereas it is likely that each individual would need two doses of the vaccine in order to become inmuno-competent. Distribution to and inside countries would probably be problematic [9] (http://www.phacilitate.co.uk/pages/phaciliate/article_fedson.html). Several countries, however, have well-developed plans for producing large quantities of vaccine. For example, Canadian health authorities say that they are developing the capacity to produce 32 million doses within four months, enough vaccine to inoculate every person in the country. [10] (http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/1114000609146_109409809/?hub=Canada)
  • Anti-viral drugs. Several new anti-viral drugs have been developed in recent years. A number of governments are working to stockpile anti-viral drugs but the work is complicated by the constant mutation of the virus, which might become somewhat resistant to some anti-viral drugs, making these drugs less effective.
  • Non-pharmaceutical means:
    • "Social distance". By travelling less, working from home or closing schools there is less opportunity for the virus to spread.
    • Respiratory etiquette. Placing one's hand in front of the mouth when coughing or sneezing can somewhat limit the dispersal of droplets.
    • Masks. No mask can provide a perfect barrier but products that meet or exceed the NIOSH N95 standard recommended by the World Health Organization are thought to provide good protection. Other well-fitting masks can be helpful but much less effective.
    • Hygiene. Frequent handwashing, especially when there has been contact with other people or with potentially contaminated surfaces can be very helpful.

Stages of a pandemic

The World Health Organization (WHO) has developed a global influenza preparedness plan, which defines the stages of a pandemic, outlines WHO's role and makes recommendations for national measures before and during a pandemic.

Most sources place the avian influenza epidemic at phase 3 although some researchers claim that it is actually moving into phase 6 [11] (http://www.recombinomics.com/News/06200502/H5N1_Pandemic_Timeline.html). The phases are defined as:

Interpandemic period

Phase 1: No new influenza virus subtypes have been detected in humans. An influenza virus subtype that has caused human infection may be present in animals. If present in animals, the risk of human infection or disease is considered to be low.

Phase 2: No new influenza virus subtypes have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk of human disease.

Pandemic alert period

Phase 3: Human infection(s) with a new subtype, but no human-to-human spread, or at most rare instances of spread to a close contact.

Phase 4: Small cluster(s) with limited human-to-human transmission but spread is highly localized, suggesting that the virus is not well adapted to humans.

Phase 5: Larger cluster(s) but human-to-human spread still localized, suggesting that the virus is becoming increasingly better adapted to humans, but may not yet be fully transmissible (substantial pandemic risk).

Pandemic period

Phase 6: Pandemic: increased and sustained transmission in general population.

Notes

The distinction between phase 1 and phase 2 is based on the risk of human infection or disease resulting from circulating strains in animals. The distinction is based on various factors and their relative importance according to current scientific knowledge. Factors may include pathogenicity in animals and humans, occurrence in domesticated animals and livestock or only in wildlife, whether the virus is enzootic or epizootic, geographically localized or widespread, and/or other scientific parameters.

The distinction between phase 3, phase 4 and phase 5 is based on an assessment of the risk of a pandemic. Various factors and their relative importance according to current scientific knowledge may be considered. Factors may include rate of transmission, geographical location and spread, severity of illness, presence of genes from human strains (if derived from an animal strain), and/or other scientific parameters.

References

  • Kuiken T et al (2004), Avian H5N1 Influenza in Cats, Science 2004 306: 241 (Template:Doi)

External links


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